1. Field of the Invention
The present invention relates to a method and a system for storing data in a fixed-length packet and for transmitting the packet, and more particularly to an ATM system.
2. Description of the Related Art
An ATM is becoming popular as one of principal techniques of current communications systems, and promises to be used wider in the future.
FIG. 1 exemplifies a network using the ATM. Here, the ATM is used for a mobile communications system. Normally, respective mobile terminals 501-1 through 501-3 wirelessly transmit/receive data (including speech data) to/from their nearest base station devices. Base station devices 502-1 through 502-3 are respectively connected to a base station control device 503, and transmit/receive data to/from the mobile terminals 501-1 through 501-3 according to the control from the base station control device 503. The base station control device 503 is accommodated by a mobile exchange 504, which is intended to exchange data within the mobile communications system. Additionally, the mobile exchange 504 is connected to an exchange network 505 in order to allow data to be transmitted and received between the mobile communications system and a different network. In the system having the above described configuration, data are stored in fixed-length packets, which are referred to as cells, and transferred between the base station devices 502-1 through 502-3 and the base station control device 503, between the base station control device 503 and the mobile exchange 504, between the mobile exchange 504 and the exchange network 505, and within the exchange network 505.
FIG. 2 shows the format of an ATM cell. Here, the format of a UNI (User Network Interface) is depicted. The ATM cell is fundamentally a fixed-length packet of 53 bytes, and is composed of a 5-byte header and a 48-byte payload. The header is composed of a GFC (Generic Flow Control) arranged for preventing cells from colliding with each other at an interface, a VPI (Virtual Path Identifier) for identifying a virtual path, a VCI (Virtual Channel Identifier) for identifying a virtual channel set on a virtual path, a PT (Payload Type) for identifying the type of information stored in a payload, a CLP (Cell Loss Priority) for indicating a priority of a cell, and an HEC (Header Error Control) for detecting an error of header data. The payload stores data to be transmitted.
Note that the format of an ATM cell of an NNI (Network Network Interface) is fundamentally the same as that of the UNI. However, the GFC is not arranged in this NNI format, and additionally, 12 bits are assigned to the VPI.
The base station devices 502-1 through 502-3 respectively have a CLAD (Cell Assembly and Disassembly) capability. The CLAD capability is a capability for assembling a cell by partitioning digital data into 48-byte blocks and by respectively attaching headers to the partitioned 48-byte blocks, and for extracting the data from the payloads of the respective cells. That is, each of the base station devices 502-1 through 502-3 stores the data received from a mobile terminal in a payload of a cell and transfers the cell to the base station control device 503, and transfers the data stored in the payload of the cell received from the base station control device 503 to a mobile terminal at a call destination. The VPIs/VCIs between the base station control device 503 and the base station devices 502-1 through 502-3 are assigned, for example, by the base station control device 503.
Upon receiving a cell from any of the base station devices 502-1 through 502-3, the base station control device 503 transfers the cell to the mobile exchange 504. Upon receiving a cell from the mobile exchange 504, the base station control device 503 transfers the cell to the base station device corresponding to the VPI/VCI assigned to the header of the cell. The mobile exchange 504 transfers the cell to a base station control device 503 or exchange network 505 based on the VPI/VCI assigned to the header of the received cell. FIG. 1 illustrates only one base station control device. Actually, however, a plurality of base station control devices are accommodated by the mobile exchange 504.
With the above described system, data transferred between mobile terminals or data transferred between a mobile terminal and a different terminal are transmitted by the ATM.
As described above, data are stored in cells and transmitted in the ATM. Fundamentally, data to be transmitted is partitioned into 48-byte blocks, and the respectively partitioned data are sequentially stored in the payloads of cells.
By the way, particularly in mobile communications, data to be transmitted is compressed in order to save network resources (such as a frequency band in this case). Accordingly, for example, the transfer rate of speech data such as a conversation, etc. is considerably low.
If respective cells are transmitted every time the payload of each of the cells is filled with the data to be transmitted in the case where such low-speed data is transmitted by using the ATM, the time intervals at which cells are transmitted become longer as a matter of course, which leads to a data transmission delay. The data transmission delay must be prevented particularly when speech or image data is transmitted, as is well known.
A method for transmitting a cell before its payload is filled with data to be transmitted is known as a method for preventing the above described delay. Specifically, for example, an interval at which a cell is transmitted is predetermined, and the data received from the transmission timing of the immediately previous cell and until the transmission timing of the present cell is stored in the payload, and data (dummy data) referred to as a pad (padding) is stored in the remaining area of the payload as shown in FIG. 3, when the cell is transmitted. The cell whose payload partially includes a pad is transmitted. The pad is, for example, data in which all of bits are 0.
As described above, if the method for transmitting a cell before its payload is filled with data to be transmitted is adopted, a delay problem can be overcome. However, because dummy data are transmitted included with data to be transmitted in this method, the transmission efficiency of data is degraded.
Although this problem is significant when the transfer rate of data to be transmitted is low, it may occur even when the transfer rate is high. Additionally, this is not a problem occurring only in speech data.
An object of the present invention is to improve data transmission efficiency in a system for storing data in a fixed-length packet and for transmitting the packet.
A data transmission method according to the present invention is performed in a system for transmitting data by using a fixed-length packet from a transmitting device to a plurality of receiving devices. This method comprises the steps of: allocating the data storage area of the fixed-length packet to one or more of the plurality of receiving devices; storing only data to be transmitted to the one or more receiving devices to which the data storage area of the fixed-length packet is allocated in a corresponding partitioned area of the fixed-length packet; storing the allocation information indicating an allocation pattern of the data storage area of the fixed-length packet in the header of the fixed-length packet; transmitting the fixed-length packet from the transmitting device to the plurality of receiving devices; and extracting data from the data storage area of the fixed-length packet based on the allocation information stored in the header of the fixed-length packet, within each of the receiving devices.
With the above described method, a transmitting device can transmit a plurality of data sets, which are unique, to a plurality of receiving devices by using a single fixed-length packet. Since the data storage area of the fixed-length packet is shared by the plurality of receiving devices at this time, there is no need to store invalid (dummy) data in order to prevent a transmission delay. Accordingly, data transmission efficiency can be improved while preventing a transmission delay. Additionally, only data to be transmitted to receiving devices are stored in the data storage area of a fixed-length packet, so that an overhead becomes smaller and the data transmission efficiency can be increased.
A data transmission method in a second aspect of the present invention is performed in a system for transmitting data by using a fixed-length packet from a plurality of transmitting devices, which are connected in series, to a receiving device. This data transmission method comprises the steps of: allocating the data storage area of the fixed-length packet to one or more of the plurality of transmitting devices; storing the allocation information indicating the pattern for allocating the data storage area to the plurality of transmitting devices in the header of the fixed-length packet; storing the data to be transmitted to the receiving device in the data storage area of the fixed-length packet based on the allocation information stored in the header of the fixed-length packet at each of the transmitting devices, when the fixed-length packet is transmitted to the receiving device sequentially via the plurality of transmitting devices; and extracting data for each of the transmitting devices from the data storage area of the fixed-length packet based on the allocation information stored in the header of the fixed-length packet, within the receiving device.
With the above described method, a plurality of transmitting devices can transmit data to a receiving device by using a single fixed-length packet. Since the data storage area of the fixed-length packet is shared by the plurality of transmitting devices at this time, there is no need to store invalid (dummy) data in order to prevent a transmission delay. Accordingly, data transmission efficiency can be improved while preventing a transmission delay.
A data transmission method in a third aspect of the present invention is performed by a system for transmitting data from a first device to a second device arranged in a network that transfers a first fixed-length packet. This method comprises the steps of: the first device extracting valid data stored in the data storage area of the first fixed-length packet when the first device receives the first fixed-length packet to be transmitted to the second device, the first device storing the extracted valid data in the data storage area of a second fixed-length packet, and the first device transmitting the second fixed-length packet to the second transmitting device.
With the above described method, data are transferred between the first and the second devices by using the second fixed-length packet which is sorter than the first fixed-length packet, so that the amount of invalid data to be transmitted can be reduced and the data transmission efficiency can be improved.